An example of an engine valve actuator is disclosed in U.S. Pat. No. 5,186,141, "engine Brake Timing Control Mechanism", issued to D. Custer on Feb. 16, 1993 (the "'141 patent"), incorporated by reference herein. The actuator disclosed in the '141 patent does not provide for engine valve seating control, although it could benefit from such control. FIG. 1 discloses the engine valve actuator of the '141 patent.
The problem addressed by this invention is to provide acceptable engine valve seating velocity in a variable valve actuation (VVA) system. Hydraulic lost motion valve actuation systems may be driven with a cam. The hydraulic displacement of an engine valve in such a lost motion system is directly proportional to the displacement provided by the cam during normal operation. In some applications, however, the engine valve must be closed at an earlier time than that provided by the cam profile. This earlier closing may be carried out by rapidly releasing hydraulic fluid to an accumulator in the lost motion system. In such instances, however, engine valve seating control is required because the rate of closing the valve is governed by the hydraulic flow to the accumulator instead of by the fixed cam profile. Engine valve seating control may also be required for applications (e.g. centered lift) in which the engine valve seating occurs on a high velocity region of the cam. Still further, engine valve seating control is required in common rail VVA designs, in which all seating events occur as a result of the release of hydraulic fluid, possibly to an accumulator.
Devices designed to gently seat engine valves have been developed in order to address the needs of systems that require valve seating control. For example, the valve catch system 100 shown in FIG. 2 was developed to provide valve seating control. The system 100 includes a slave piston 120 disposed within an actuator housing 110. The slave piston 120 is slidable within the housing 110 so that it may open an engine valve (not shown) below it. A screw body 130 extends through the top of the housing 110 and abuts against the slave piston 120 when the latter is in a resting position (i.e. engine valve closed). A plunger 140 is disposed within the screw body 130 and is biased towards the slave piston 120 by a spring 160. The screw body 130 may be twisted into and out of the housing 110 to adjust engine valve lash.
The plunger 140 serves to selectively limit valve seating speed velocity as the slave piston approaches its home position (engine valve closed), thereby allowing the engine valve to close more gently than it otherwise might. The plunger 140 is mechanically limited from extending beyond the screw body 130 by more than a preset distance .delta., thus allowing the slave piston 120 to return rapidly until contacting the plunger, within .delta. of the valve seat.
The system 100 operates under the influence of hydraulic fluid provided through a passage 150 in the housing 110. During the downward (valve opening) displacement of the slave piston 120, hydraulic fluid flows through the passage 150 in the housing 110 and through the passages in the slave piston so that the slave piston is forced downward against the engine valve. During the upward (valve closing) displacement of the slave piston 120, the hydraulic fluid flows back through the passages in the slave piston 120 and out of the passage 150 in the housing 110. As the slave piston 120 approaches its home position, it forms a seal with the plunger 140. The seal between the plunger 140 and the slave piston 120 results in the building of hydraulic pressure in the space between the slave piston and the end wall of the housing 110 as the slave piston progresses towards its home position. The building hydraulic pressure opposes the upward motion of the slave piston 120, thereby slowing the slave piston and assisting in seating the engine valve.
While the valve catch system 100 shown in FIG. 2, which works on slave piston pressure, has achieved acceptable valve seating velocity over a wide range of engine speeds and oil temperatures, improvements are still needed. For example, the valve catch system 100 tends to hold the engine valve open longer than is desirable for optimum engine breathing at high engine speeds. The system is also prone to reduce valve velocity to nearly zero prior to seating and thereafter accelerate the valve so that it seats at an unacceptable velocity. This type of valve catch system also may require a complicated slave piston design, which increases high-pressure volume, increases the length and flow resistance of the fluid path between the slave piston and the passages leading to the master piston, trigger valve, or plenum, and increases the required slave piston height and weight. Increased high-pressure volume may be detrimental to compliance. Increased flow path length and flow resistance produce increased pressure, whih may also be detrimental to compliance. Additionally, increased pressure drop may make it difficult to maintain master piston pressure greater than ambient during periods of decreasing cam displacement of high engine speed, which may allow air bubbles to form in the oil. Another difficulty that may be experienced with the valve catch system 100 is increased viscous dissipation, which may increase oil cooling load and parasitic power loss.
The valve catch system 200 shown in FIG. 3, which works on valve catch plenum pressure, is considered to have lower parasitic loss than the system shown in FIG. 2. The system 200 includes a slave piston 220 disposed within an actuator housing 210. The slave piston 220 is slidable within the housing 210 so that it may open an engine valve (not shown) below it. A screw body 230 extends through the top of the housing 210 and abuts against the slave piston 220 when the latter is in a resting position (i.e. engine valve closed). A plunger 240 is disposed within the screw body 230 and biased towards the slave piston 220 by a spring 260. The screw body 230 may be twisted into and out of the housing 210 to adjust engine valve lash. A fluid passage 250 through the housing 210 leads to a master piston (not shown) and/or a trigger valve (not shown).
The system 200 operates similarly to the system 100 shown in FIG. 2, except that in system 200, the hydraulic pressure that opposes the upward movement of the slave piston 220 is built inside the screw body 230. Although performance may be improved using the system 200, compliance difficulties may still be encountered due to the high pressures required and the increased compliance associated with the smaller area of plunger 240.
The embodiments of the present invention distinguish over the valve catch systems 100 and 200 shown in FIGS. 2 and 3. The various embodiments of the present invention include a variable area orifice in the system plunger. The embodiments of the invention have reduced compliance especially during decompression braking, higher master piston pressure during periods of decreasing cam displacement at high engine speed, reduced parasitic power loss and consequently reduced VVA housing cooling load, and reduced slave piston length and weight as compared with the valve catch system shown in FIG. 2. Furthermore, the embodiments of the innovation have reduced peak valve catch pressure as compared with the valve catch system, shown in FIG. 3. The variable flow restriction design in the invention is expected to be more robust than the constant flow restriction design with respect to engine valve velocity at the point of valve catch engagement and oil temperature and aeration. The variable flow restriction allows the displacement at the point of valve catch/slave piston engagement to be reduced, so that the valve catch has less undesired effect on the breathing of the engine.